I can't remember if Scrot was/is included in i3. Sorry.
Read all of this before you make a decision...You can try installing Scrot by opening your Terminal and entering...
sudo apt-get install scrot
Then try using the Prtscrn key. If it's successful, it will place the picture in your Home folder. Open the File Manager, and you will see it in addition to all of the other folders that are visible. If that doesn't work, it will take a key bind to make PrtScrn take a screenshot, and since I'm not on i3, I'm not sure what that keybind would be or where you should put it. Sorry.
IF...and I repeat...IF I remember correctly, i3 also uses the file manager PCManFM. If you install the Xfce4 Screenshooter (as outlined above), afterwards, you should be able to open PCManFM and click the "Applications" option in the left-side panel. Then click on "Accessories", and it should be there.
Again, this is the long way around, and I'm not on i3...so take all of this into consideration before you proceed. Perhaps an i3 user will chime in soon.